Conventionally, a radar system in a vehicle detects preceding vehicle by transmitting a laser beam or the like. The transmitted laser beam is reflected from a reflective surface on the preceding vehicle, and is returned to be captured by the radar system. In this manner, the radar system can detect the preceding vehicle at a distance of more than 100 meters. However, the signal noise ratio (S/N ratio) of the laser beam reflected from the reflective surface decreases when the reflection from the preceding vehicle is deteriorated. That is, a signal factor in the reflected laser beam from the preceding vehicle and a noise factor in the reflected laser beam from other objects are indistinguishable. As a result, a detection range of the radar system decreases
Various methods for increasing the S/N ratio of the signal used in the radar system have been proposed. That is, for example, a radar system having a signal integration function for an improvement of the S/N ratio of the signal from a detected object is disclosed in Japanese patent document JP-A-2004-177350. The radar system in this disclosure uses a polygon mirror that rotates at a constant speed for reflectively transmitting a transmitted wave. Each of the transmitted waves transmitted in a different direction scans a predetermined range of angles for detecting an object. The radar system integrates the signals reflected from the detected object for an improvement of the S/N ratio.
However, a problem is experienced in the radar system disclosed in Japanese patent document JP-A-2004-177350 because the transmitted wave cannot be directed in the same direction more than once due to the use of the rotating mirror having a constant rotation speed. That is, for example, the transmitted wave cannot be projected toward the same part of the preceding vehicle plural times within a predetermined interval. Further, the transmitted wave in a horizontal direction is scattered over an angle that is much greater than a required range for object detection because of the mechanical characteristics of the rotating polygon mirror. Therefore, the radar system has to cancel transmission of the transmitted waves toward unnecessary angle ranges. As a result, time for transmission of the transmitted waves for unnecessary angle ranges is wasted. In other words, reduction of wasted time is desirable for improvement of S/N ratio.